This invention regards a device for and a method of fixing and lifting vertically installed pressure tanks in ships. In particular, it regards vertically installed pressure tanks, in which two or more pressure tanks are arranged in a cassette, and in which the cassette is arranged to be lifted out of and into the ship's cargo hold.
Sea transport of gaseous petroleum products essentially has taken place by means of the so-called Liquefied Natural Gas (LNG) method. The method comprises cooling of gas to a liquid state, whereupon the gas may be transported in ship tanks at atmospheric pressure. The method requires costly equipment both at the shipment and receiving locations. As the gas must be cooled to a relatively low temperature, up to a fifth of the gas is used to operate the cooling and heating processes. Consuming this amount of energy solely in transport-related processes is expensive and also environmentally questionable.
Several other ship-based solutions have been proposed, in which the gas is pressurised and/or cooled in order to achieve a gas density that is practical for the purpose. Such solutions have gained little practical use, but a solution in which a large number of vertical tubular pressure tanks are placed in the cargo hold of a ship, has attracted considerable attention. The method is termed PNG—Pressurised Natural Gas. According to such a method, the gas is compressed at the shipment location to an overpressure of a couple of hundred bars, and the gas then is filled onto the pressure tanks located on the ship. The cooling is limited to a simple and inexpensive removal of the gas compression heat, so as to leave the transport temperature near ambient temperature.
By installing relatively long cargo pressure tanks vertically in cassettes, it is possible to better exploit the material properties of the cargo pressure tanks, whereby the net weight of the cargo pressure tanks relative to the loading capacity of the ship may be reduced.